Prospects for Life on Sub Neptune-Sized Exoplanets

The search for atmospheric biosignature gases as signs of life on exoplanets is gaining momentum. So far, however, there are only a handful of rocky exoplanets suitable for atmosphere study with next-generation telescopes. To broaden prospects, we describe the possibilities for life on sub Neptune-sized temperate exoplanets (those receiving Earth-like irradiation from their host stars). Sub Neptune exoplanets are both common and favorable for discovery and observational characterization. Compared to their super-Earth counterparts they have larger radii and their lower atmospheric mean molecular mass means their atmospheres are more extensive. Yet, sub Neptunes with extensive atmospheres or gas envelopes lack any solid surface as we know it. This has motivated us to evaluate the possibility of an aerial biosphere. We also consider temperate sub Neptunes with cold enough interiors to have a liquid water ocean beneath the atmosphere and atop layers of supercritical water and high-pressure solid ice. We describe prospects for life and signs of life on sub Neptune exoplanets, life both in the clouds and in a water ocean.